Dry Final Product

3.1 Pilot Testing

Preliminary data taken in the laboratory regarding the separation characteristics of a product can be beneficial when beginning the equipment selection process. If one is retrofitting an existing unit with an identical system, it will not be as complex and time consuming as designing a "grass roots" facility. Pland data can then be helpful. However, should the process be altered, one should evaluate the effect on centrifugation.

Careful attention should be taken to ensure that the existing tank design, peripheral pumps, piping and agitators do not provide shear that will cause particle degradation. Capital dollars spent in this area on crystallization studies, the selection of the correct pumps, etc., will directly impact the capacity of your equipment as particle degradation will significantly affect throughput and final residual moisture adversely, as it does filtration. (See Ch. 8 for a more detailed discussion.)

If a process is existing and in-plant expertise is available to optimize the equipment, it would be advisable to do so before a purchase. Upon review of the existing design, sufficient improvements can often be made in an older piece of equipment thereby avoiding a more costly investment. Vendors usually offer this type of assistance at no charge from their office or at a daily rate in the field.

3.2 Data Collection

The first step is to collect pertinent information to the process, including a process flow sheet, product information and completion of a typical questionnaire, as shown in Table 2.

Knowledge of the most critical aspect of the process can guide the sometimes difficult selection process. For example, the requirement of a very dry product with strict impurity levels suggests a filtering centrifuge. A product with a feed rate of 150 gpm, without wash requirements, would lead us to a continuous sedimentation centrifuge.

A simple Büchner funnel test will indicate fast, medium or slow filtration. Slow filtering materials that have inordinate quantities of particles passing the filter paper will be submicron and difficult to capture in a filtering system. Therefore, a sedimentation centrifuge should be considered.

A phenomenon called "cake cracking" can occur and will be evident in this simple test. Not all materials exhibit this. It depends upon the surface tension of the product and its tendency to shrink as dewatering occurs. Amorphous, thixotropic materials will exhibit this more than rigid solids.

Table 2. Product Questionnaire: Centrifuges, Filter Press. (Courtesy Heinkel Filtering Systems Inc.)

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